The present invention relates to the art of light transmission in wave guides and specifically to a flexible fiber-optic light guide, briefly called a fiber optics of the type including a plurality, e. g. 10 to 10.sup.6 thin fibers or filaments each consisting of a highly transparent core, e. g. made of glass, fused silica or plastic, and each being provided with a uniform coating of a material having a lower refractive index than the core. Typical coating substances are transparent materials from the broad category of inorganic glasses and organic polymers. For brevity, the combination of such a fiber core with a cladding or coat is referred to as a "conductor" while the assembly including a plurality of fibers is referred to as a "bundle".
The use of such fiber-optic conductors and bundles as well as their production is described in the patent literature since 1926 starting with British Pat. No. 285,738 to J. L. Baird and U.S. Pat. No. 1,751,584 to C. W. Hansell. A resume of the art is to be found in a publication in "Applied Optics and Optical Engineering", edited by R. Kingslake, Academic Press New York 1967, Volume IV, pages 1-29, by W. P. Siegmund "Fiber Optics" incorporated by reference into this specification.
Generally, the core portion of such fibers will have a diameter in the range of from about 10 micrometers to about 100 micrometers while the coating thickness is in the range of, for example, 1 or 2 micrometers. As the refractive index of the coating is smaller than that of the core, any light entering the interface between core and coating will be reflected back into the core rather than entering into an adjacent conductor.
Bundling of the conductors is most critical in the end portions of a flexible fiber-optic device in view of optical continuity, i. e. arrangement within an optic system in which the fiber-optic device is the transmitter of an optical continuity from one of its ends to the other. As an example of such an optical continuity or system, a light source may be near one end of a fiber-optic device used for illuminating an area near its other end, or an image formed by a conventional optical lense system may be transmitted from one end of the device to its other end for scanning, viewing, and the like. The bundle of the device may be branched so as to have one uniform end portion at its receiving or emitting end and two or more end portions at the opposite receiving or emitting end. Such fiber-optic devices may further have separate bundle portions for different functions, e. g. one portion guiding light from a light source near a first end of the device to its opposite second end for reflection, or absence of reflection, upon an object near said second end, guiding the reflected light into said second end, then through another bundle portion to said first end and evaluation of the light signal thus produced by means of a sensor near said first end. In all such systems, the ends of the fiber-optic device are adapted for optical continuity in the sense of being capable to receive and/or emit the light or image. This includes but does not require direct contact. A conventional adaption of the ends of the device for optical continuity is effected, for example, by cutting, grinding, or polishing an end portion so as to produce a substantially planar face thereon, preferably normal to the fiber axes. When such an end portion includes a substantially rigid sleeve, casing or the like, it can be said to constitute a socket and this term includes any type of end piece or head piece of the fiber-optic device to be arranged in optical continuity with another element of the system in question, e. g. a light-source, a light-sensor, a light-receiving area, a light-emitting area, an image-forming area, an area of image processing (scanning), etc. Direct contact of the bundle end in the socket with such other elements is not required though some type of sockets may be used for contact coupling, e. g. between the end faces of adjacent in-line connected bundles.
As is well known, fiber-optic bundles can be used for image-transfer systems if the fibers are properly aligned at both ends, whereas no alignment of the fibers is required when only light -- not an organized image -- is to be transmitted. As will be explained below, proper alignment of the fibers in a bundle may impose limitations as to the diameter of the fibers.
One of the essential characteristics for various and sometimes contradictory purposes is flexibility of the bundle at least in a portion of its length. In general, flexibility of the conductor as well as of the bundle increases as the diameter of the individual coated fibers or conductors decreases, and a suitable sheath or hose of a flexible material will be used to surround the bundle for protection. For many potential applications of fiber optics, e. g. replacement of a conventional image transmission via lenses, mirrors and prisms by a bundle of flexible light conductors for endoscopy, as well as for light transmission between moving parts of a machine and a stationary apparatus, and for many other purposes where a more or less curved path of transmission (light or image) is advantageous, usefulness of the fiber-optic device will depend, in part, upon its flexibilty.
Flexibility can conveniently be expressed in terms of the smallest possible bending radius of the bundle or the device, respectively. Such minimum bending radius of a fiber optics in the form of a conductor bundle is generally determined by three essential parameters: (a) the mechanical and physical properties of core and cladding material, (b) the diameter of the conductor, and (c) the distance between the conductors.
As a general rule, flexibility of the conductors and the bundle increases and the bending radius obtainable therewith decreases, as the bending strength of the conductors (combined properties of core and cladding) increases and as the diameter of the conductors decreases. Also, a fiber-optic bundle can be bent without breakage of the conductors only if the conductors are in a mutually sliding relation, e. g. by providing that the distance between these conductors is sufficient so that substantial portions of the conductors are free to permit relative motion. On the other hand, such distances between the conductors must be sufficiently small so that the mutual guidance of the conductors prevents that individual conductors are bent more than the bundle and thus are subject to breakage.
For many uses including illumination, signal or image transmission, etc., permanently bent portions would be desirable near one or more of the bundle ends and it may be most advantageous to arrange such permanent bend or curve of the bundle within a head-piece type socket. Such curves may have a homogeneous curvature (i. e. the curve radius remains substantially constant throughout the bend) and may include angles of from about 180.degree. to about 30.degree. or less.
Practical experiments made in connection with this invention indicate that such rigid bends with small radii of curvature are subject to unavoidable and relatively fast aging even when the device is produced with the greatest possible care. Even in the absence of shock-impact one, or mechanical stress in, the permanent bend, some conductors will break spontaneously after a certain period of time. As a consequence, the cross-section of the fiber bundle effective for transmission of light or images will decrease as the age of the fiber-optic device increases.
These problems of the permanent bend or curve present an even more severe limitation in the production of curved or bent portions of fiber optics where the conductors include a core made of fused silica, i. e. the so-called "quartz fibers". The bending strength of quartz or fused silica is less than half the bending strength of normal glass. In addition, some practical reasons of commercial production tend to limit the use of very thin conductors as would be desirable for achieving small radii of curvature in the rigid bend: Mutual coordination or alignment of conductors required for transmitting an image becomes more complicated and time consuming as the numbers of conductors to be assembled and aligned in a bundle having a predetermined cross-sectional area increases. For this reason among others, conductors with a relatively large diameter are preferred for producing image-transmitting bundles. Further, the number of reflections of transmitted light at the interface between core and coating of a fiber increases as the core diameter decreases, and at a bent or curved interface there is a tendency that impinging light will not be totally reflected but will emerge from the fiber. As higher losses of the transmitted light may ensue as the number of reflections per unit of length increases, thinner conductor cores may become impractical thus limiting the bending radius. Finally, fibers with a core made of fused silica frequently will be coated with plastics and the separate coating step required will increase the costs per length of such fibers. As a consequence, fiber-optic devices having cores of fused silica will be less costly if the required effective cross-section of the bundle is filled with conductors having larger diameters.